Cell Structure and Membrane

• Cell structure and the cell membrane are discussed
• Dr. Nehal Ibrahim, a lecturer of Biochemistry presented the material

Eukaryotic Cell Structure

• A cell is composed of protoplasm
• Protoplasm includes the nucleus and cytoplasm
• The three main parts of a cell are the cell membrane, nucleus, and cytoplasm
• The cytoplasm contains cytosol, a soluble fraction, and cell organelles

The Cell Membrane

• The cell membrane, also known as the plasma membrane, is a biological membrane
• It forms the outer layer of a cell
• It separates the cell's interior from the outside environment
• In animal cells, the cell membrane solely establishes this separation
• In plant cells, fungi, yeast, and bacteria, there is an additional cell wall for protection and support

Functions of the Cell Membrane

• Acts as a barrier to protect cells
• Regulates the movement of substances in and out of cells via a semi-permeable barrier
• Provides shape to the cell
• Receives chemical messengers for cell-to-cell communication
• Attaches cells to each other to form tissues

Constituents of the Cell Membrane

• Primarily composed of lipids and proteins
• Carbohydrates make up less than 10%
• In eukaryotic cells, lipids and proteins are nearly equal in weight, with a lipid-to-protein ratio close to 1
• Carbohydrates are attached to lipids and proteins

Membrane Lipids

• Three main types exist: phospholipids, glycolipids, and sterols
• Phospholipids are the major type, contributing to >50% of all lipids in plasma membranes
• Glycolipids account for only a small amount, about 2%
• Sterols make up the rest
• All three types are amphipathic, containing both hydrophilic and hydrophobic groups

Phospholipids

• Mainly composed of a glycerol backbone, two fatty acids, phosphoric acid, and another group
• They have a polar hydrophilic head, made of glycerol and phosphate
• Non-polar hydrophobic tails are made of two fatty acids
• The cis double bond creates a kink formation in the chain

Lipid Bilayer

• Functions like a fluid
• Phospholipid molecules can diffuse freely within the plane of the membrane
• Hydrocarbon chains also move

Glycolipids

• Composed of a glycerol/alcohol backbone, fatty acids, and a sugar part replacing the phosphate group
• In nature they are amphipathic
• Fatty acids present are hydrophobic
• Sugar parts are hydrophilic
• Glycolipids are exclusively in the outer layer, with carbohydrates extending to the outside
• Carbohydrates are involved in cell-to-cell communication

Cholesterol

• Present in animal cell membranes
• Absent in plants
• Amphipathic
• Contains a steroid ring and a non-polar hydrocarbon tail

Cholesterol Functions

• Immobilizes the hydrocarbon tails of phospholipid molecules
• Decreases the fluidity of the plasma membrane
• Confers rigidity and strengthening effects

Membrane Fluidity

• Phospholipids and proteins can move or diffuse freely in the plane of the membrane
• Results in the membrane being fluid-like
• Types of movement include rotation, swinging, lateral movement (lateral diffusion), and transversal movement (flip-flop)

Factors Affecting Membrane Fluidity

• Cholesterol: Increases in cholesterol decrease fluidity
• Saturated and unsaturated fatty acids: Unsaturated chains increase fluidity
• Unsaturated fatty acids create kinks, preventing tight packing by the fatty acids
• Leads to increasing fluidity in the membrane

Abnormalities of Cell Membrane Fluidity

• Includes Spur cell anemia
• Hemolytic anemia form
• Characterized by abnormally shaped RBCs, known as spur cells
• Occurs in severe liver diseases, such as alcoholic cirrhosis
• It Increases cholesterol in RBC membranes
• Leads to decrease cell membrane fluidity
• Results in spiny-shaped RBCs instead of the normal biconcave shape
• Causes premature destruction of cells in the spleen
• Produces serious effects on cells' ability to pass through capillaries

Membrane Proteins

• Embedded in the lipid bilayer
• Can move laterally, but cannot flip flop
• Two types exist: intrinsic/integral and extrinsic/peripheral
• Integral proteins reside within/span the membrane
• Peripheral proteins loosely adhere to the membrane, either inside or outside

Integral or Intrinsic Proteins

• Transmembrane proteins
• Types & functions include transport proteins, channel proteins, and carrier proteins
• Transport molecules and ions across the membrane
• Also include Proton pumps and receptors

Peripheral or Extrinsic Proteins

• Loosely adhere to the membrane
• Types & functions include enzymes and identification proteins such as antigens
• Also involved in cell communication and signaling

Carbohydrates

• Short chains only present on the outer surface of the plasma membrane
• Attach to proteins, forming glycoproteins, or to lipids, forming glycolipids

The Fluid Mosaic Model

• Describes the plasma membrane as a fluid combination of phospholipids, cholesterol, proteins, and carbohydrates